Local correction of cable signals



March 1., 1938. H. F. WILDER lLOCAL CORRECTION 0F CABL SIGNALS n Filed April 21, 1937 2 sheets-sheen 1 RMI@ .2.49. QRDGQQ vv www All H. F. WILDER LOCAL CORRECTION OF CABLE SIGNALS March l, v1938.

Filed Apri1r2l, 193'? 2 .Sheets-Sheet',` 2

jme/MM lyll Planned Mu. 1, 193s PATENT OFFICE.

Loon. coanac'rIoN or CABLE sromms' namur F. wilder.' aIoomI'IeIs, N-.'J., minor tn The Western Union Telegraph Company, Ney1 York, N. Y., a corporation 'of New York Y Application AprilZl, 1937, Serial No. 138,262 '1 calms. (cI. 11s-70) '.l'his invention relates to ocean cable telegraph systems andl pertains particularly to-circuit arrangements for restoring the signal waves to their -4 proper shape by supplying through a local' cor- -jlthe telegraph signals which are suppressed in .-,lters and ampliilers of the primary receiving Eequipment.

, .,It is well knownthat'telegraph signals are at- 1 0A tenuatedand distortedin their passage through a'cable. due to the resistance, capacity' land inductance of the cable. In recent years the ef,

in transmission, but the signal is nevertheless quite suitable for the control of retransmitting or printing equipment. It is necessary, however,

-25 to pass the signals through shaping. networks and. amplifiers before they enter the receiving apparatus. Due 'to the loss oi low frequency components in the shaping networks and the amplitler, the signal waves aredistorted or are not 30 sustained in amplitude and if the signal frequency becomes sufficiently low. the current through the receiving impedance may approach \zero`and` the entire generated E. M. F. will be'-y come stored on thecapacity. 35 'Ihe object of my inventionis to restore the signal waves to their normal undlstorted condition without the presence of low frequencydisturbance by returning to the signal wavesl an E. M. F. of the proper shape having frequency com- 40, ponents equal in magnitude but opposite in vphase tothose components suppressed by the local capacitance.

In the following' descrIpumi of my Invenn'an Il f shall refer to the accompanying drawings, in

` .45 Ivhich-4 Figures 1, 2, 3 and 4' are diagrams illustrating conventionally ;the circuit arrangements vof. equipment embodying my invention which may be employed at the receiving end oi' a-cable. 60 Figure 5 illustrates the' graphs of voltages developed by'rthe signal waves and the voltages de-l veloped in the local correction circuits andshow- 1 ing the' .manner of combining, these voltagesdto produce a perfect signal wave for retransmission. 55 The receiving end of the cable is connected to 5 rection circuit the low frequency components of the yduplex bridge and to the shaping networks in the manner shown in my prior application Serial No. 90,804,.filed July 15, 1936 and are well understood by engineers. The manner of determining the characteristics oi the cable and the 5 proportions' of the shaping and amplifying-elements vat the terminal are set forth in said prior application. Y

In order to electrically separate the cable from the amplifier and its-associated circuits and also to suppress low frequency interference or induction, the amplifier is coupled tothe shaping networks by a transformer illi. To suppress high frequency induction and interference due to natural potential difierencesbetween the terminals l5 l of the cable and from mis-matching oi the cable and artificial line impedances under duplex conditions. as pointed out in said application, a low pass network is introduced between the shaping networks. These same sources may produce v' low frequency will appear at the `receiving Vter- 30. Y minal. upon which thew signals are superposed,

thereby causing' the latter to deviate from their vs o-called-zero position. These 'adverse frequencies oi a low order' may be' effectively suppressed or` prevented from entering the receiving apparatus by the interposition of a condenser or,a properly adjusted high passfilter. The1at-' tenaiso suppresses' low frequency components in Ythe signal waves, so that the longer signal pulses are not sustainedgin amplitude and fall to zero, 40 with consequent distortion and susceptibility to .interferencexas the E. M. F. declines to zero. I

It becomesnecessary. therefore. to locally `restore the missing low frequency components thus Y suppressedf in order to 'reform-the signal to its 45 original condition. r,The o bJectot my invention isto provide in ieans embodying thermionic vacuum tubes for thus 'locally restoring these -missf .ing components to the receiving equipment.

For the purposeV of explaining my method of restoring the missing components by local `correction. I willrefer to the circuit ,arrangement`- of Fig. 1,v and the explanatory diagrams of Fig.

5.v A two-'stage vacuum tube signal-'shaping telegraph amplifier shown. The amplier is connected to the shaping network and cable by means of the transformer I0. A resistance capacitive tween the stages of the amplifier. By means of an extra biasing winding I2, on the retransmitting relay I2, the D. C. component of the plate current of the vacuum tube 2, is neutralized, to allow the relay to be freely operable by the signal A. C. component.

Assuming that a negative signal pulse modulates the grid oi' vacuum tube I., the plate current Im will be decreased, thereby causing point a to become more positive, since the battery B1 is opposed by a smaller IR drop in resistance Rx.. The consequent positive charging current into condenser Ca and to ground through resistance Ri will raise the potential ofthe grid ofvacuum tube 2, causing a positive pulse of current to flow through the operating coil of relay I2.

The signal voltage wave developed across the resistance R1 is of the form shown at Fig. 5-a. Resistances Rz and Ra are made equal and constitute a potentiometer, dividing the signal voltage approximately in half. The signal voltage developed between the points g and b is thus l shown at Fig. 5-b. The operation of the arma.-

ture of relay I2 applies a voltage from battery to the input terminals of the local correction network. Ultimately this potential, assumed to be 7.5 volts, will appear .across the resistance Rs but the rate at which the maximum is reached is controlled by the electrical constants of the correction network to be an inverse function of the decay of the tail of the arrival curve. This local correction (abbreviated CXN) voltage developed between the points c and d is symbolically shown at Fig. 5-c.

'I'his local correction network voltage transient will in turn develop a potential of 2.5 volts across R1, 2.5 volts across Rz and 2.5 volts across Ra and the sum of the drops across R1 and Rz is 5 volts which is,` in the-steady state condition, exactly equal to the maximum peak deflection of the signal transient, also betweenpoints g and b'. The voltage transient developed between the points g and b' by the local correction network is represented at Fig. 5-d. Hence the combined effect of the signal voltage at point 0' with the local correction voltage at said point g produces a resultant operating pulse which is maintained at steady state value throughout its extent, as illustrated at Fig. 5-e.

The operation of my local correction system of cable signals as above described is theoretically correct. In practice it has been found helpful to shunt the resistance R; with a series capacityresistance network R4 C: to compensate for the eii'ect of the initial charging current to the condenser Cs through the tube' plate impedance Rpi and the tube load resistance Rr. in parallel. As the local correction voltage begins to appear across R1, the effect ofthe condenser C: and the preceding tube plate circuit is to lowerI for a brief charging interval, the eii'ective resistance between ithe points (lf-b. The shunt R4 C: is adjusted t0 .lust compensate for this interval by passingV to l ing a local correction to the received cable sig'- nais whereby the low frequency components suppressed in the primary receiving equipment are restored to the signal waves, with particular reference to the circuit arrangement of Fig. 1. The

operation of the other modifications shown in A Figs. 2, 3 and 4 will be obvious without further detailed description.

Certain types of four-elementv high mutual conductance vacuum tubes, the UX48 for example, oii'er the desirable possibility of very efiicient and powerful operation of a magnetic relay when the coils of the latter are connected as in the push-pull output circuit of Flgure.2. The local correction current is supplied by modulating the grids of the tubes by means of the symmetrical grid circuit and low pass filter.

In cables of less attenuation anl amplifier having tubes of the type above mentioned in pushpull arrangement, may be, employed and connected as indicated in Fig. 3. In this case a transformer Il is introduced between the local correction network and the potentiometer grid bridge circuit. This arrangement also permits the station generator I0 to be utilized as a supply both for the push-pull tube circuit and for the local correction network. The transformer may also be equipped with a tertiary winding for the purpose of introducing an opposing oscillatory voltage to compensate for undershoot in the signal'as is sometimes desirable under conditions of severe interference ln the high frequency end of the signal spectrum.

In Fig. 4, I have shown another modification loi? this invention which maybe advantageously employed` under certain conditions. 'I'hus if the ratio of transformation of the transformer is great the high internal impedance of the sec-l ondary may prevent the loading of the latter transformation yof unity and the'introduction of this impedance is, therefore, negligible.

In order that the low frequency correction currents shall pass through the correction transformer Il without distortion, the ratio of the primary inductance of the transformer to the resistance of the primary winding, plus the reflected' secondary load resistance, plus the resistance of the correction network shunted across the primary, should be kept high as possible. Ratios of L/R of 4 and 6 have been found satisfactory for the majority oi' signal circuits. A transformer so constructed is large physically but quite practical and is in general use in cable signaling equipment.

In order to more completely disclose this invention I have shown several circuit arrangements which may be employed to suit different conditions but it will be evident to engineers that various other modifications may be made and that any combination of the circuits for amplifying and correcting may be made to best suit :the power sources available, without departing from this inventionor from the scope of my claims.

i. In a communication system having a receiving terminal providedwith signal shaping and amplifying means which distort the received signals by suppressing the lower frequencyv components of the received signal impulses, the method ofrestoring to normal amplitude the dislof 'a sinar impulse bythe conjoint action jo ,sein

distorted impulser and said generated impulse.

2. In a communication system havin'git` receiving terminal provided with signal shaplngaridv sists in locally generating a voltage impulse, shaping and developing said impulse as an inverse function4 of the decay'A of the distorted received signal impulse, .and retransmitting an unponents of the/received signal impulses, the method of restoring to normal amplitude the distorted portions of the signal impulses, which `consists in initiating the local generation of voltagesby the operation of the received distorted signal impulses', shaping said generated'voltages `in linverse ratio to the decay of said distorted voltages,

combining the distorted and locally generated voltages to produce resultant sustained voltages and electrostatically applying said sustained voltages to control the transmission I' of undistorted signals.

4. vIn a' communication system having a rect-.fiv-l ing terminal provided withsignalsliaping and amplifying meanswhich distort 'the received signals icy-suppressing the lower vfrequency components of the received signal impulses, a repeating or relaying apparatus embodying a circuit arrangement havinga thermionic vacuum tube provided with anode, cathode and grid elements, a current source, a relay for repeating or relaying signal impulses from said source to a recorder or transmission line, said relay having its operating coil in the output of said tube, means for producing a correction voltage having characteristics which are an inverse function of the decay of the distorted received signal voltages, and means to subject the grid of said tube 'tc the conjoint control of the received distorted'voltage and the l`correction voltage., whereby-the repeating relay transmits undistorted signals. 5. Av repeating orrelaying apparatus as set forth in` claim 4, said correction means compris- `nega. low pass -filter network having character- `fs'tics'which produce; voltage transients embodyng components V,corresponding to the low frequency components o! the received signals sup- ',press'ed in the shaping and amplifying means.

f- Av"6.Ifn a communication system having a receivy ,ing terminal provided with signal shaping and amplifying means which distort the received signals `by suppressing the lower frequency co'mgponents of the received signal impulses',v` `the method of restoring to normal amplitud'ethedistorted portions of the signal impulses, which con-f amplifying means which distort the received signals by suppressing the lower frequency components of the received signal impulses, a resists ance connected across the output of said amplifying means, a repeating or. relaying circuit connected in shunt to said resistance and having a pair of high resistance. potentiometers, a pair of thermionic vacuum tubes having anode, cathj ode and grid elements connected at mid-points respectivelyof said potentiometers and in inverter arrangement, means for' locally generating voltages in synchronism and phase with the...

received signals and embodying components corresponding to the suppressed components ofthe received signals, means for subjecting the grids of the tubes 'tothe conjoint control of the volti a recorder or transmission line.

'7.,In acommunication system having a recelvj ing terminal provided with signal shaping and amplifying means which distort the received sig-fl Vnais by vsuppressing the-lower frequency components of the received signal impulses, a 'repeating or relaying arrangement connected to the output. of said amplifying means embodying a vtransformer and a pair of thermionic vacuum tubes having anode, cathodeand grid elements, said tubes being connected in inverter arrangement with their grid elements connected to the lsecondary of the transformer, a source of electrical energy, a rcorrection network having characteristics whic transmit only frequency corn-l ponents corresponding to said suppressed components of the received signals, means for connecting said network to saidsource in synchronism with the received signals,I means for subjecting said tubes to thel conjoint control of the received signals and the output of said network,- and means'in the output of the tubes for repeating signals into a recorder or transmission line@ HAROLD F.` wnnna. 

